Hermetic compressors of the type used in refrigeration systems usually comprise, in the interior of a casing, a motor-compressor assembly having a cylinder block within which is defined a cylinder having an end closed by a cylinder head defining, therewithin, a discharge chamber in selective fluid communication with a compression chamber defined inside the cylinder and which is closed by a valve plate provided between the closed end of the cylinder and the cylinder head, said fluid communication being defined through suction and discharge orifices provided in said valve plate and which are selectively and respectively closed by suction and discharge valves generally carried by the valve plate.
During the compression of gas, heat is generated as a result of different processes, such as: the heating of the gas during compression; the losses due to attrition on the bearings, where the power by viscous attrition is transformed into thermal energy and heat; and the losses in the electric motor, which are also transformed in heat.
In its constructive form, the compressor is mounted in a casing connected to the refrigeration system which includes, besides the compressor, a condenser, an evaporator and an expanding device. This circuit is hermetically sealed, not transferring mass to the external ambient.
One part of the thermal power generated by the compressor is sent with the refrigerant fluid to the discharge line and dissipated in the condenser of the refrigeration system. The other part is transferred to the refrigerant fluid and to the lubricant oil contained in the interior of the casing. On their turn, the refrigerant fluid and the lubricant oil transfer the other part of the heat to the casing, which dissipates said other part of the generated heat to the external ambient.
This system achieves a thermal balance when certain conditions are maintained constant, such as for example the temperature of the external ambient and the operating condition of the compressor, considering as constant the evaporation and condensation pressures and the ventilation characteristics.
In this situation of thermal balance, a temperature profile can be established, which is directly related to the energetic efficiency of the compressor, since, on one hand, the heating of the ambient of the casing causes heating of the lubricant oil, reducing its viscosity and the power that is lost by viscous attrition. The load capacity of the hydrodynamic bearing is dimensioned taking into account this viscosity reduction. On the other hand, there are many negative aspects resulting from the heating within the casing, such as: temperature increase of the refrigerant fluid being drawn; compression power increase resulting from the high temperature of the cylinder; and the need to use special materials in the construction of the compressor to resist the high temperatures.
The usual process of heat transfer from the inside to the outside of the compressor presently occurs as follows: the heat generated in the compression of the refrigerant fluid is transmitted to the cylinder block and to the discharge muffler and then it is transferred, by convection, to the gas in the internal ambient of the compressor and also to the oil falling on said heated surfaces. The gas and the oil will change heat with the internal walls of the casing and the heat will have to trespass the wall of the casing by conduction, to be finally dissipated, by natural or forced convection, from the compressor body to the external ambient. In this process, there is a series of thermal resistances that impair heat exchange and heat dissipation.
There are also known from the art the following heat transfer processes: by forced ventilation occurring between the internal components and the lubricant oil and between the compressor body and the ambient outside its casing; and by cooling the lubricant oil through a cooling pipe, through which the refrigerant fluid of the condenser of a refrigeration system to which the compressor belongs is deviated to a heat exchanger immersed in the oil inside the compressor, removing heat therefrom.
The known prior art presents different alternatives to promote heat transfer, such as: using heat exchangers with Stirling machines, as taught in patent U.S. Pat. No. 6,347,523; providing fins in the cylinder heads and an auxiliary air circulation system; using heat pipes; using a fluid pumping system by means of pumps driven by oscillatory, mechanical and electrical movements, etc.
However, such known solutions present some disadvantages. In the case of the known solutions which use finned cylinder heads and heat exchange with air, the disadvantage resides in the fact that it is not possible to achieve high heat transfer capacity. In said systems, a saturation limit in relation to the heat transfer capacity is easily achieved. This occurs as a function of the saturation of the efficiency of the fins by increasing the length of and/or decreasing the distance between the fins, or by the impossibility of finding air moving equipments with sufficient capacity to allow reaching the pressure and flowrate levels which are required in determined heat transfer capacities. Moreover, such solutions lead to an increase of vibrations and noise in the refrigeration system and to less reliability due to the large amount of movable parts they have.
In a known solution disclosed in patent U.S. Pat. No. 6,499,977 a scroll compressor carries, in its exterior, a refrigeration system using a heat pipe. In this solution, the heat in the compressor casing is removed by means of a heat pipe system. Heat transfer is improved only from the external surface of the casing to the external ambient, maintaining constant the other thermal resistances. Such compressor has a constructive characteristic in which the cylinder is directly exposed to the external ambient and therefore the high thermal resistance of the gas of the internal ambient does not cause any damages to said compressor. However, for the reciprocating hermetic compressor it is highly desirable to minimize or eliminate such internal thermal resistance of the gas.
Another solution of heat transfer by using heat pipes is disclosed in patent U.S. Pat. No. 6,412,479, in which the heat pipes are provided in the interior of an internal combustion engine to remove heat from the cylinder head. Nevertheless, said solution refers to an internal combustion engine (and not to a hermetic compressor) in which the objective is to re-use the unburnt gases of the discharge in the supply system.
Other known solutions described in patents U.S. Pat. No. 5,651,258 and U.S. Pat. No. 5,695,004 also present a heat pipe system for removing heat from the interior of the compressor, re-using or not said heat in a refrigeration system to which the compressor is associated. Such solutions however are not directed to the issue of energetic efficiency of a hermetic compressor, since the heat pipes are applied to the system to use said heat and not to remove it from the hot parts of a hermetic compressor.